Patent GB-1108701-A (GB)
Title: Communication switching system
Inventor: ABBOTT HENRY HERRICK
IPC: H04Q3/54
Filed: 1965-07-28
Abstract: 1,108,701. Automatic exchange systems. WESTERN ELECTRIC CO. Inc. 28 July, 1965 [31 July, 1964], No. 32161/65. Heading H4K. A PABX employs an end-marked reed-relay switching network 100 on the line side of which are terminated line circuits LC10-LC89 and on the trunk side of which are terminated outgoing trunks 130-1, 130-n, incoming trunks 132-1, 132-n, and the originating and terminating ends of intercommunication trunk circuits 134-1, 134-n, for internal calls, a group of registers 102-1, 102-n, having appearances on both sides of the network; the exchange having a common control circuit 102 responding to demands for service from the line circuits, trunk circuits and registers, and being governed by a mode circuit which suits the control function to the demand; each line circuit having a gate, such as G10, controlled by the associated subscriber's station, register, and common control circuit to establish line conditions and network marking potentials. Line circuits.-The gate, such as G10, in each line circuit comprises a transistor-diode network (Fig. 23, not shown) which normally extends the substation loop circuit over contacts of a cut-off relay to leads G and F, which terminate in the common control. The line circuit is identified and accessed by two tens and two units code leads which are selectively marked from a common control circuit scanner 106. The line circuits may also be accessed over the same code leads by a number register such as 123-1 present in each register 102-1 to 120-n and similar to the circuit scanner 106. The gate circuit provides signals indicative of line circuit state and sends these to the common control over a pair of leads shown symbolically as SL10. Control wires S and C switched with the speech wires over the network also control the gate G10. Scanner.-The scanner 106 comprises a 3 x 4 4 units counter followed by a 3 x 4 tens counter each counter producing a code in which 1 out of 3 and 1 out of 4 leads is not marked. The scanner is normally disconnected and stands in a reset state providing a code such that when the scanner is connected all line circuit gates have at least one code lead marked. Scanning for a calling condition. Potential on the G and F leads hold gates G10 on to mark the line circuits idle. Off-hook from the substation overrides the G and F leads to switch G10 off and produce a signal over SL10 to set the common control in a so-called line dial tone mode in which a calling line is connected to a register. To this end the scanner 106 is connected in its reset state and since one of the code leads to the calling line circuit will be marked it causes the gate circuit G10 to switch on, a signal over SL10 being thereby sent to the common control which sets the scanner stepping in search of the calling line. When the scanner reaches the code corresponding to the calling line circuit all four code leads to the circuit are at earth and the gate G10 switches off producing a signal over SL10 to stop the scanner. In this condition of the gate circuit a transistor driven relay operates in the line circuit to extend a mark over the C-wire to the network. Trunk and register selection.-The registers, intercommunication trunks, outgoing trunks, and attendants' trunks, are combined as one group served by a single scanner. If any class of trunk or the registers are fully occupied a circuit available logic indicates the fact without need of scanning. If at least one of the wanted circuits is free the common control directs the scanner to search for and halt on the first idle one. The scanner is a 3 x 4 x 2 counter. Connectimi of register.-When seized on command of the common control a register marks its C-wire to the trunk side of the network. With marks present from the line circuit and the register the network responds to provide a connection between them. Ground on the sleeve conductor S from the register operates the cut-off relay in the line circuit which disables the off-hook detection path to common control and restores the function of the G and F leads so that gate G10 is switched from off to on. In switching the gate a signal on path SL10 instructs common control to reset the scanner 106. The register supplies dial tone and receives dialled digits. Resistor, capacitor timing circuits detect inter-digital pauses and steer the digits for registration in tens and units 3 x 4 counters 123-1 providing the same coded output as the scanner 106 in common control. Connecting called line to trunk.-With a called number registered the register applies to the common control by way of a register bid circuit 110 for a call termination procedure involving the control in a read-register mode. When available the control removes supervision of the line circuits to mask the appearance of service requests; this masking is effected by the G and F leads. The line circuit code leads are connected to the register and the wanted line circuit, if free, responds to the code by switching its gate circuit from off to on, the consequent signal to common control on lead SL causing the trunk group to be scanned for an idle intercommunication trunk circuit which when seized marks its C-lead in its call terminating appearance on the trunk side of the network. A similar mark is present in the called line circuit consequent on its interrogation over the code leads and the network responds by setting up a connection to the trunk. Connecting calling line to trunk.-Preparatory to its release the register clears the digits of the wanted line and extends a mark over the sleeve conductor S to the calling line circuit to switch its gate circuit off, all line circuit gates having been switched on by the control circuit by way of the G and F leads. The change of state in the calling line circuit brings the common control scanner into action the line circuit gate switching on when presented with the scanner reset state and switching off again when presented with its own code in the presence of a mark on the sleeve conductor S. When halted on the calling line circuit the control releases the register and governs the line circuit and intercommunication trunk to place marks on their C-leads to the network so as to effect a connection. The trunk circuits supply ringing current which is continuous for an initial period timed by a capacitor after which it is interrupted. Ringing tone is produced by a capacitative coupling of ringing current round the trunk feed bridge transformer. The trunk releases the connection in response to calling line on-hook, the called line hook switch having no effect. Busy conditions.-If the called line is found to be engaged or if no suitable trunk circuit is free for the connection, the common control seizes a busy tone trunk, of which a number are terminated on the trunk side of the network, and directs the register in a calling line call back procedure to connect the busy tone trunk and release the register. If no busy tone trunk is available the register is retained to provide a busy tone from its own resources. Calls to central office, operator, and dead numbers.-The register detects digits 9 and 0 for central office calls and calls to the operator, respectively, the central control and register co-operating in the call back procedure to replace the connection between calling line and register with a connection between the calling line and the appropriate trunk circuit. When a dead number is dialled the register code counters are stepped to the end, when the common control finds no response from the line circuits, with the result that the call is routed to the operator. Dilatory dialling.-An RC timing circuit in the register causes the register counting circuits to be stepped to their end position if dialling is incomplete or unduly delayed. The register then responds as though the digit 0 was dialled and the call is connected to the operator. Calls from central office.-These are handled by the operator who picks the calls up from lamp signals and, given the wanted number on the PABX, operates a start key to split the line and make a bid for common control circuit service in its trunk dial tone mode. In the trunk dial tone mode the common control seizes a register and marks its appearance on the line side of the network which, with a mark extended from the calling trunk, produces a connection over which dial tone is sent. The operator dials the wanted number on receipt of which the register recruits common control to cooperate in a test of the wanted line. If it is busy or the number is unequipped the register sends busy tone. If it is idle the register and common control cooperate in a call-back procedure to identify the calling trunk so that the register may be released and a connection set up between the calling trunk and called line. The operator may retire on reply or during ringing. Should the operator dial 0 or 9 the register connects busy tone.